How do infants learn to produce the sounds of their language? The vocal abilities of infants change dramatically over the first year of life. Beginning with the earliest, immature vocalizations, infants make rapid progress, typically producing their first words by 12 months of age. Along the way, they begin to produce speech-like syllables and to structure sequences of syllables in accordance with the phonological rules of their language environment. While the vocal achievements of the first year are well-described, not nearly as much is known about the mechanisms of change that drive vocal development. Most work focuses on the maturation of the vocal tract, but studies of vocal development in songbirds found that there are also social sources of developmental change. For example, young male cowbirds (Molothrus ater) rely on the reactions of females to shape their immature sounds into functional song. Based on the avian work, the investigators'preliminary studies have shown that infants can learn new patterns of vocalizing from caregivers'reactions to their babbling. How do infants use social feedback to create new, more developmentally advanced, vocalizations? The goal of the proposed research is to understand the mechanisms by which infants incorporate the phonological patterns of their language into their vocal repertoires. Based on preliminary studies, the investigators hypothesize that the contingent responses of caregivers to babbling facilitate infants'statistical learning of the phonological patterns of their language. To investigate this hypothesis, the variability and temporal contingency of speech to 9-month-old infants will be manipulated to assess their effects on vocal learning. The proposed research has important implications for educating parents in providing optimal learning environments for their infants. An understanding of the role of socially guided learning in speech and language could be used to help parents to be more sensitive to their infants'behavior in ways that would facilitate development. Investigating social influences on phonological development can also contribute to the study of speech-language pathology and of processes underlying both successful and disordered communicative development. PUBLIC HEALTH RELEVANCE: By illuminating mechanisms by which infants learn to produce the sounds of their language from caregivers'contingent speech, the findings could inform interventions for disordered language development. The results could also be used to help parents and child care providers create social environments that foster and support language growth. Eventually, this research could be used to design preventive programs for infants with a higher risk of language delay (e.g., children with Down Syndrome or SLI).